1. Field of the Invention
The invention relates to a method and arrangement for measuring a liquid flow in connection with a pump system. The invention is preferably applied to pump systems in which the pump is driven by an alternating-current motor, whose rotation speed is controlled by a control unit, such as e.g. a frequency converter.
2. Description of the Related Art
Pump systems are used in the industries and in public utility services, among other things. In industrial applications, pump systems are in most cases used in connection with production processes, while they relate to transfer of pure water, rain water and waste water in municipal engineering. In conjunction with pump systems, it is often necessary to know the momentary liquid flow and the liquid amount transferred over a given period. Flow data are needed for several purposes. Flow data allow supervision of the condition and operation of the pump and of the functionality of the liquid transfer system. Flow data allow detection and localisation of e.g. leaks and obstructions in the piping or pumps of the liquid transfer system. Flow data are also useful in the billing of liquid transfer. In production processes, flow data are needed for controlling and monitoring the process.
Pump systems used for liquid transfer usually consist of one or more electrically driven pumps. The electric drive consists of a suitable power supply circuit, an electric motor and a control unit suitable for controlling and/or adjusting this. The pump operates as a load on the electric drive. The most frequently used electric motor in pump systems is an alternating-current motor, especially a induction motor. The control unit used in an alternating current motor often consists of a frequency converter because of the benefits gained by this. The speed of the electric motor is adjusted by the frequency converter, which converts the frequency of the voltage supplied to the motor. The frequency converter, again, is adjusted by appropriate electric control signals.
A prior art pump system is illustrated in FIG. 1. The pump 140 is actuated by an electric drive consisting of a power supply 101, a frequency converter 120 acting as the control unit and alternating-current motor 130, which in this case if a three-phase current motor. The motor is usually connected to the pump with the rotation speed of the motor and the rotation speed of the pump being identical. The power supply 101 comprises an alternating-current network, such as a three-phase network, or the like, for supplying electric power from the alternating-current source to the electric drive. The liquid flow through the pump is measured in the system of FIG. 1 by means of a separate flow meter including a flow sensor 151 and a measurement unit 152 equipped with a display.
The flow sensor may be e.g. an ultrasonic sensor or a mechanical flow sensor. However, a “pressure-difference sensor” is used in most cases, this sensor measuring the pressure difference generated by the flow in the flow direction and in the direction opposite to the flow. The flow Q can then be determined by the following formula:Q=k*√{square root over (Δp)}  (1)in which k is the constant determined by the flow path between the pressure sensors and Δp is the measured pressure difference.
However, the use of a separate flow meter involves a number of drawbacks. Very high precision is required from a sensor for determining pressure difference in order to achieve such flow measurement precision that is adequate for ordinary applications. The use of such sensors thus incurs considerable costs. In addition, the mounting of a separate flow meter causes work at the mounting site, and the suitable mounting site and arrangement for the flow meter will often have to be planned separately each time. The mounting site conditions may also vary, and hence flow meters of different types will have to be used depending on the mounting site conditions. These factors increase the overall mounting and equipment cost.